Support traced placeholder for images

[stefanjudis]

Traced placeholders are a very nice way to improve the user experience for images that are still loading. It would be awesome to have these. 🎉

[axe312ger]

I love this feature so much and would freak out to see it working with gatsby + contentful.

Issue here is: The tracing has to run on the local machine since contentfuls image api does not support this. We would need to patch the contentful source plugin to support that feature.

It would have to work somehow this way:

1. Download image from Contentful image API to the local machine
2. Run the trace from gatsby-plugin-sharp on these images
3. Extend the query fragments to contain these new traced images as base64

🚀 🎉

[axe312ger]

Fixed in #26